76 BERTRAM G. SMITH 



eggs of Necturus, Plethodon, Spelerpes and Desmognathus, which 

 are also laid under cover, are likewise unpigmented. 



A very thin and transparent' 'vitelline membrane'^ — the zona 

 pellucida of the ovocyte — closely invests the egg; it is quite 

 inconspicuous in fresh material. This is not the true cell wall of 

 the egg, which, as described in detail on page 112 lies immediately 

 within the vitelline membrane and represents in a modified form 

 the zona radiata of the ovocyte. 



Proceeding from within outward, the coverings of the egg may 

 be enumerated as follows: (a) the cell wall; (b) the vitelline mem- 

 brane lying in close contact with the preceding; and (c) the cap- 

 sule or thick gelatinous outer envelope, which is separated from 

 the vitelline membrane by a space filled with fluid. 



During the first few hours after fertilization the capsule gradu- 

 ally becomes turgid by osmosis, becoming in this way a much 

 more efficient protection to the egg; the space between the egg 

 and its capsule is increased by the absorption of water and in 

 this the egg almost floats, resting lightly on the lower inner sur- 

 face of the capsule. When the eggs are removed from water the 

 egg proper looks much larger than it really is, because magnified 

 by the spherical capsule. 



For a day or two the envelopes are quite soft and somewhat vis- 

 cous, making it rather difficult to cut them with scissors in order 

 to remove the eggs. Gradually the material of the envelopes 

 becomes firmer. The connecting cord is at first quite elastic, but 

 it loses this quality to a considerable extent after prolonged immer- 

 sion in water. 



Until after the eggs have been in water for several days, the 

 outer layers of the envelopes are still cast into wavy folds or 

 wrinkles, usually extending spirally about the capsules and the 

 connecting cord. As a rule the spiral is constant in the direction 

 in which it extends about the axis of the string in all portions of 

 the cord and capsule. These spiral folds are usually most strongly 

 marked at the ends of the cord adjacent to the capsule, and here 

 they often persist (fig. 1), suggesting the chalazae of the hen's 

 egg. 



